The disclosure relates to oxidation-resistant ferritic steel compositions that can be used as a separator for solid oxide fuel cells (SOFC) and for other applications where high temperature stability and oxidation resistance are required.
As seen in the prior art FIGURE, a typical planar solid oxide fuel cell stack 10 includes one or more cells 12 comprised an electrolyte plate 18 sandwiched between a fuel electrode (anode) 14 and an air electrode (cathode) 16. The electrolyte plate 18 is typically formed of the stabilized zirconia, e.g., yttrium stabilized zirconia. A separator 20 (also referred to as an interconnect) is typically used in a lamellar-like structure to separate multiple cells 12 in order to attain a usable power supply. H2 and CO2 are supplied as fuel sources to a passage space 22 formed between the separator 20 and the fuel electrode (anode) 14. Air is supplied to another passage space 24 formed between the separator 20 and the air electrode (cathode) 16. The electrolyte plate provides oxygen ion conductivity to facilitate the reaction between the hydrogen and the oxygen ions so as to generate electrons.
The solid oxide fuel cell is formed as a lamellar structure of plates in order to reduce the internal resistance and to increase the effective electrode area per unit volume. The coefficient of thermal expansion of the material employed for the separator 20 is preferred to be close to those of the air electrode 14, the fuel electrode 16 or the solid state electrolyte 18; better corrosion resistance and high conductivity are also required for the materials used in the separator 20. Typical separators are formed of electrically conductive ceramic materials such as (La, alkaline earth metal) CrO3 based materials
If the surface area of the electrolyte plate 18 is formed to be larger than those of the fuel electrode 14 or the air electrode 16, the lamellar forming can be easily achieved with the separator 20, resulting in easily holding the electrolyte plate 18 in place. However, since the separator 20 is often made of brittle ceramic materials such as the LaCrO3 materials mentioned previously, there remain problems including insufficient strength, delamination, and poor formability.
Accordingly, the separator is clearly an important component of SOFCs. Its primary function is to serve as a support for the electrolyte, anode and cathode; separate cells, seal hydrogen gas (H2) and air as fuel sources, and at the same time to permit the flow of electrical current. Because of this, the separator must be formed of a material that has electrical conductivity at high temperature, e.g., greater than 600° C.; be oxidation resistant within the SOFC operating environment, and have an equivalent coefficient of thermal expansion as the electrolyte (e.g., yttria stabilized zirconia). However, ceramic materials are relatively expensive as well as difficult to fabricate since they are inherently brittle materials, especially for the larger SOFCs currently being fabricated.
Recent efforts have been made to replace the ceramic materials with metal or steel based alloys. Metal or steel based alloy materials, therefore, require several important parameters be met for fuel cell components including, among others, a strong resistance against oxidation, desirable electro-conductivity, and thermal cycling stability when operated in oxidation and fuel areas within the high temperature environment. When such a metallic material is used at up to 1000, degrees Celsius, the metallic material oxidizes and an oxide film is formed on the surface. Ideally a metallic material used as a separator for a fuel cell would allow an oxide film to form with a desired thickness and then remain stable at that thickness arresting subsequent oxide formation and at the same time providing the desired electrical conductivity. Current ferritic based steel compositions typically show high growth rates and resistivities of surface oxides formed during high temperature exposure. In fact, the area specific resistivities (ASR) of these compositions are projected to be in excess of 150 milliohms-cm2 after 40,000 hours at 850° C. The oxide thickness after such as an exposure is expected to be in excess of 30 microns. Oxide thicknesses in excess of 30 microns are likely to delaminate during SOFC operation and crack during thermal cycling.
Accordingly, there continues to be a need for improved steel compositions that exhibit increased oxidation resistance, lower resistivity, have a coefficient of thermal expansion that closely matches that of the electrolyte as well as thermal cycling stability since the standard operating lifetime of the SOFC is typically rated for 40,000 hours or more.